Vibration calibrator



w. A. YATES ErAL f June 25, 1957 VIBRATION CALIBRATOR 4 Sheets-Sheet 1Filed Sept. 30. 1954 mmv NVENTORS WILFRED A. YATES ARTIN VlDS N BY M D OATTORNEYS June 25, 1957 w. A. YATEs ETAL VIBRATION CALIBRATOR 4Sheets-Sheet 2 Filed Sept. 30. 1954 INVENTORS WILFRED A YATES BY MARTINDAVIDSON ATTORNEYS June 25, 1957 w. A. YATl-:s ETAL VIBRATION CALIBRTORYY 4 sheets-sheet s Filed Sept. 30, 1954 REG.

INVENTORS W|LFRED A YATES BY MARTIN DAVIDSON 29. z; zMfMZ/ ATTORNEYSJune 25, 1957 w. A. YATl-:s ETAL 2,796,756

VIBRATION CALIBRATOR Filed sept. 3o, 1954 4 sheets-sheet 4 INVENTORSWILFRED A. YATES MARTIN DAVIDSON BY ATTORNEYS 2,796,756 Patented June25, 1957 VIBRATION CALIBRATOR Wilfrid A. Yates, Silver Spring, andMartin Davidson,

Bethesda, Md., assignors to the United States of America as representedby the Secretary of the Navy Application September 30, 1954, Serial No.459,562 5 Claims. (Cl. 73-1) (Granted under Title 35, U. S. Code (1952),sec. 266) The invention described herein may be manufactured and used byor for the Government of the United States of America for governmentalpurposes without the payment of any royalties thereon or therefor.

The present invention relates to a wide range calibrator for vibrationmeasuring apparatus and more particularly to a novel vibrationcalibrator for accurately measuring and indicating the peak vibrationamplitude of a vibration generator or standard.

The importance of determining the vibration characteristics of variouselements of systems in use today is universally recognized. Hence, anumber of vibration measuring arrangements have been devised formeasuring and indicating the frequency, amplitude and other vibrationcharacteristics of such Vibrating elements or systems.

Since each measuring arrangement employs a vibration pickup ortransducer of one type or another to translate the mechanical motioninto electrical or other suitable media, it is customary to calibratethe pick-ups with a vibration standard. In the past, the calibration ofthese vibration standards created no problem since vibrationmeasurements were made infrequently and were generally limited to roughapproximations. However, with the ever-increasing and wider applicationsof vibration measurements, a serious calibration problem is introduced Iand the need for a rapid, reliable calibration of a vibration standardover wide ranges of amplitude and frequency is emphasized. This isespecially so in instances Where the higher frequency components ofvibration must be determined and where resonant properties of thepick-up may complicate analysis.

Accordingly, it is one of the prime objects of the present invention toprovide a vibration calibrator for a mechanical vibration standard whichis capable of rapid, reliable calibrations of the vibration standardover wide ranges of amplitude and frequency.

Another object of the present invention is to provide a vibrationcalibrator which provides an extremely accurate indication of the peakvibration amplitude of a vibrating body.

A further object of the invention is the provision of a vibrationcalibrator which provides a direct meter indication of vibrationamplitude over a wide range of frequencies.

A still further object of the invention is the provision of acalibrating instrument employing a mutual-inductance transducer toobtain precise vibration measurements.

Another object of the present invention is the use of the invarience ofthe pick-up transfer characteristic under static and dynamic conditionsto standardize the instrument for high frequency vibration measurementsby means of a static displacement.

Another object of the present invention is the provision of a vibrationcalibrator employing a pick-up which is non-sensitive to the dielectriccontstant of an intervening medium. y

Yet another object of the invention is the provision of a vibrationcalibrator which is easily adaptable for use with various types ofvibration generators and which may itself be standardized by acomparison involving a displacement which is easily obtainable in alaboratory.

Still a further object of the present invention is the provision of arelatively simple apparatus which is exceedingly stable and accurate forperforming its intended duties.

Other objects and many of the attendant advantages of this inventionwill be readily appreciated as the same becomes better understood byreference to the following detailed description when considered inconnection with the accompanying drawings wherein:

Figure l is a perspective view of a vibration calibrator constructed inaccordance with the teachings of the present invention and positioned inoperative relation with a conventional vibration generator which is tobe calibrated;

Figure 2 is a block diagram view of the Various components comprisingthe preferred embodiment of the invention shown in Figure 1;

Figures 3a and 3b are schematic diagrams which-When joined together at ashow a complete circuit diagram comprising the preferred embodiment ofthe invention;

Figure 4 is a cross-sectional view taken on the lines 4--4 of Figure land showing the constructional details of the mutual-inductancetransducer or pick-up head;

Figure 5 is a side elevation of the pick-up head shown in Figure 4; and

Figure 6 is an enlarged cross sectional view showing the details of theinductive pick-up head of Figure 4.

Referring now to the drawings wherein like reference charactersdesignate like or corresponding parts throughout the several views,there is shown in Figure 1 a perspective View of a vibration calibrator10 embodying the preferred features of the instant invention, and avibration generator 12 which is to be calibrated. The vibrationcalibrator, which is basically adapted for use as a laboratoryinstrument, primarily comprises two basic components. These componentsinclude a vibration pickup unit or transducer 14 of the non-contactingdisplacement-type and a main unit 16 comprising the electrical andelectronic circuits thereof. Generally, vibration calibrator 10 operatesto accurately measure the vibration amplitude and the operatingfrequency or range of frequencies of the vibration generator 12. Uponaccurately determining the vibration characteristics of the vibrationgenerator over its entire operating range, the generator is used as alaboratory or manufacturing standard to directly calibrate the unknownmeasuring characteristics of various transducers or vibration pickups.'Ihat is, the vibration meter reading of the vibration calibrator iscompared with the electrical output of a transducer mounted on thevibration generator. After being calibrated the transducer may be usedelsewhere to make vibration measurements. Preferably, the electricalcomponents of the vibration calibrator 10 are enclosed in a housing 17having a plurality of indicating instruments and control means fordetermining and controlling the operation thereof. These instruments andcontrols will hereinafter be more particularly describedy and identifiedwhen reference is made to the specific circuit arrangement of thevibratorgenerator.

Referring specifically to Fig. 2., there is shown a complete blockdiagram of the vvarious electrical components comprising the vibrationgenerator 10. p This generator includes the mutual inductance transducer14 comprisingA tion generator 12 under calibration, transformer 18cornprises a variable mutual inductance unit.

The carrier oscillator '26 serves as a source of excitation for theprimary winding 21 of the pick-up and thus yacts to generate analternating current at a predetermined frequency. Preferably, the outputfrequency of the oscillator is in the order of several megacycles 'suchas, Vfor example, 2.3 megacycles. This vcurrent is fed through theprimary Winding 21 of transformer 18 for excitation Vof the pick-up 14and to the primary winding 27 of a fixed mutual inductance transformer28 of a regulator circuit 29. As will hereinafter become apparent, thevoltage output VC of the transformer 28 is proportional to the productof the current and frequency in its primary winding. This voltage isused to'regulate the output current and frequency of the carrieroscillator through 'the regulator circuit 29 in that the regulatorVcircuit 'serves to provide a correcting feedback signal to voscillator26 which is proportional to an error signal from the oscillator.

It will be apparent that as surface 24 is rapidly vibrated, the radiofrequency output 'of the pick-up secondary '22 will be modulated 'at thefrequency of vibration and vbecause the Aexcitation of the'prirnarywinding isheld constant by regulation, the output of 'thesecondary 22will be 'a funtcion of the separation d only. This output is fed to thecarrier detector 20wherein the modulated radio' scale in the circuit 'of'the A. C. voltmeter is calibrated in terms of peak values to provide adirect amplitude indication. This calibration arises from 'thestandardization of the instrument when the vibration generator isquiescent and is accomplished by.means of a chopper operating iny its A.C. excited mode i. e. the moving contact 218 of the chopper beingrapidly switched back and forth between the stationary contacts at, forexample, 60 cycles per second and by means of a calibrate-zero circuit33. In this circuit a voltage V is developed with which the carrierdetectorV D.-C. output voltage Vs, which is present whenever the pick-up14 is in its zero-set position, is compared by means of the chopper. Thecalibrate zero circuit is adjusted so that V0 is equal to Vs and hencethese voltages buck out when the pick-up is at'the zero set position.vThen as will be more'specically described,'by adjusting the pick-uptoward surface 24 `a vprecisely measured standardizing distancefrom the'zero-set position and adjusting the A. C. voltmeter and other circuits,a direct comparison may then be made between the amplitude of surfacevibration and an actual 'static displacement of the transducer orpick-up 14.

The specific circuit arrangement ofthe vibration calibrator is 'shown inFigs. 3a and 3b; the two gures being joined together at the terminalsa-a to form a unitary unit. For convenience in the description of thecircuit below, the circuit is vdivided into individual functionalcomponents and each'will be separately described. For purposes ofclarity, the individual components are also separated in the drawings bya series of .dotted line sections. Itis to be understood,'however, thatthese units mutually coact with each other to 'form an integral systemfor performing the functionsof the vibration calibrator.

Carrier oscillator VThe carrier oscillator 26 functions to fprovide'anexcitation current to the primary winding ofzprobe. 14 and comprises anelectronic tube 36, preferably of the tetrode Y type. The oscillatorfurtherfincludes a tank'circuit Ycouplate element of tube 36 on theother end. A conventionaltank capacitor 37 is connected in paralleltherewith. Plate-to-grid feedback for sustaining oscillation in thecircuit is obtained through a plate transformer 38 which has its primarywinding connected to a regulated B-lpotential by way of line 39 andresistor 41. The other terminal of the primary winding of transformer 38is connected to the plate element of tube 36 and to the tank circuitthrough a coupling capacitor 42. The secondary of transformer 38 has oneterminal connected to ground and its other terminal connected to thecontrol grid element of tetrode 36 through a grid leak resistor V43which is connected in parallel with'a by-pass capacitor 44. The cathodeelement of tube 36 is directly connected to ground. In a known manner,oscillator 22 Will generate in its tank circuit a current having afrequency whichY is determined by the Vparticular values of inductances21 and 27 and capacitor 37. This frequency ispreferably xed at severalmegacycles such as for example, `2.3 megacycles.

Regulator The circulating current and oscillating frequency oftheoscillator tank .circuit is closely regulated by regulator 29. Thiscircuit performs its regulating functions by including the carrieroscillator in a feedback loop Whose 'gain is controlled by an errorsignal from the oscillator. For this purpose, Vfixed mutual inductancetransformer 28 is coupled to the tank circuit of the oscillator. This`transformer which has .its primary winding `27 in the Atank circuit ofthe oscillator, develops a signal potential Vc in its secondary `windingwhich is proportional to the current and frequency .of the tank circuit.This potential is rectifiedV in a circuit including diode 46 .andsubtracted from'a reference D. C. potential. In so doing, the diodeserves as a peak detector.

The reference D. C. potential is obtained through line 47 from asuitable regulated B-lpotential Vsource Vof approximately 105 volts andacross va precision divider ycircuit comprising resistors 48 and 49 .ofwhich resistor -48 has one terminal connected to the regulated potentialsource and resistor 49 has Vone terminal 4connected to ground potential.The common'terminal of resistors 48 and `49 is connected throughresistor 51 to the plate element of diode 46. -It will be .apparentVthat by placing a regulated potential upon the plate element of diode46, conduction yof the diode `Will'occur .only when an error signal isdeveloped; that .i-s, when a difference lof potential exists between therectified signal and the reference potential. This error signal lis thenfiltered in Va resistancecapacitance lter 52. -It is -n-oted thereforethat the error signal is developed by rectifying the voltage in theYdiode circuit and subtracting 'this -voltage from the reference D. C.voltage maintained Yby 'the precision vdivider consisting of resist-ors48 and 49.

This error signal yis fed from the detector circuit through line `54to-a two stage D. C. voltage amplifier including electronic amplifiertubes 56 and 57, -both pref-V erably yof Jthe triode type. The circuitslof these tubes are basicallyidentical andinclude a pair of loadVresistors 59 `and 61, repectively, which are connected through avoltage :dropping resistor 63 to a regulated B-lpower supply -line 39and VVto a R. F. bypass capacitor '62 to ground. The-cathode lelementyof'tube 56 is .grounded as at 64, While the `amplified `error signal`output of this tube `is taken from its plate element and directly fedto the control grid element ofampliier tube 57. Thevcathode element lofampli'ertube t57 is biased positivelyby a resistor 65 connected Ithereto`and to apositive potential of 'approximately v. lDesirably avby-passf.capacitor 66 is .connected across resistorl65 Iand toY ground.p

The signal output V-ofitheamplier stages is taken from the plate elementof tube 57 and'fed tothe controlgrid y `elements yofl an electronictriode tube 68 of a cathode follower stage. The plate element of tube-68is directly connectedxto al'B-]-Y supply'fpotentialof approximately400 v.

while the cathode element is connected to the screen grid of oscillatortube 36; the screen grid Aalso being connected to ground through aby-pass capacitor 69. The signal output from the cathode follower stageis thus fed to the screen grid of oscillator tube B6 and as such, actsto control the circulating current 4and frequency of the oscillator tankcircuit.

As will hereinafter become apparent, any variations in the secondarywinding of transformer 28, which are indicated as changes in the signalVc, will be indicated by the A. C. voltmeter 32 as a variation in themeasured amplitude of vibration. By providing the closely regulatedcarrier oscillator wherein the product of the circulating current andfrequency in the oscillator tank circuit is maintained constant, theexcitation of the pick-up l14 is maintained constant within the desireddegree of accuracy.

In order to increase the gain of the second stage of D. C.amplification, a resistor 71 is connected between the cathode element ofcathode follower tube 68 and the cathode element of amplifier tube 57.The resistor 71 serves to provide a positive feedback signal to increasethe amplification of the second stage. Also connected to the cathodeelement of tube 68 is a capacitor 72. The other terminal of capacitor 72is connected to the control grid of the first -stage amplifier tube 56.The purpose of capacitor 72 is to prevent free oscillation of theregulation loop by restricting the frequency response of the amplifier.By utilizing capacitor 72, a negative feedback is introduced and theloop gain is reduced by 1one-half at a frequency of 80 cycles persecond. It will be noted that the regulation loop improves the signal tonoise ratio in the oscillator circuit, which, in the present embodiment,determines the lower limit of vibration amplitude measurement. The noiselevel in the present embodiment produces an approximate .4 microinchdeiiection on the voltmeter.

Pick-up circuit The transducer or pick-up 14 is la mutual inductancetype of electronic micrometer and generally comprises a R. F. excited,air-core transformer in which the coupling between the stationaryprimary and secondary windings varies with respect to the distancebetween the plane of the windings and a metallic surface. As shown inFigure 3b, the electrical circuit of pick-up 14 comprises a pair ofmutually inductive windings in which the primary winding 21 is connectedin series with a winding 27 and comprises a portion of the oscillatortank circuit. The secondary winding 22 has one end grounded and theother terminal connected to a suitable detector 73 such as, for example,a germanium crystal diode detector. It will be noted that the detectorloading of the secondary winding is made small so that a substantiallyopen circuit condition prevails, and, thus, the secondary wind-ing doesnot materially affect the operation of the oscillator tank circuit. Thedetector circuit further includes a coaxial cable 74 of relatively lowcapacitance and a resistor 235 (Il-iig. 3a) connected in paralleltherewith.

Pick-up 14 is sensitive to vibration by virtue of the fact that upon themovement of adjacent metallic surface 24, the inductive coupling betweenthe R. F. energized primary and secondary windings varies. This effectsan amplitude modulation of the carrier output of the secondary winding.It is obvious that with a constant current and frequency excitation ofwinding 21, the open circuit voltage output of the secondary windingwill be a function of the distance between the metallic surface and thetransducer only. While a variation in the effective inductance of theprimary winding, due to the proximity of the metallic surface, willeffect the frequency of oscillation by a small amount, the R. F.excitation which is a product of the primary current and frequency, willremain constant. The preferred structural embodiment of the pick-up isdisclosed in Figures 4, 5, and 6 and will be described hereinafter.` Theoutput from the probe' is selectively fed to the calibrate zero voltagecircuit 33 and the decade A. C. `arnplier on a fifty percent duty cycletime sharing basis or continuously to the decade A. C. amplifier 30depending upon the particular arrangement or operation being performedat that time. While the calibrate zero voltage circuit will -bedescribed hereinafter, it is preferable to first describe the decade A.C. amplifier for purposes of simplicity.

Decade A. C. amplifier The decade A. C. amplifier includes apreamplifier which is used to amplify the signals from the probe whenmeasurements are made by the lowest scale of the instrument and a mainamplifier 82 which is used to amplify all signals arising from thetransducer or pickup. The preamplifier, which has a gain of very closeto l0 comprises two stages of amplification and a cathode followeroutput stage. The first amplification stage cornprises an electroni-ctube 85 of the triode type. As will become apparent, the signal is fedfrom the secondary winding 22 of the pick-up 14, through a filterswitching arrangement to be later described, and to the control gridelement of tube 85 through a lead 86.

The cathode element of tube 85 is grounded through a resistor 87 whilethe plate element is connected through a loading resistor 88 and avoltage dropping resistor 89 to a regulated B+ potential ofapproximately +300 v. The output of the first `amplification stage istaken from the plate element of tube 85 through a resistance-capacitancecoupling circuit comprising capacitor 91 and grid resistor 92 to thecontrol grid element of an electronic tube 93 of the secondamplification stage. The cathode element of tube 93 is grounded throughbiasing resistor 94 while the plate element of tube 93 is connected toB-lpotential through a plate loading resistor 96 and the voltagedropping resistor 89. The amplifier signal output of the secondamplification stage is then fed to a cathode follower output stagecomprising an electronic tube 97 through -a resistor network includingresistor 98 and resistor 99. The signal output from the cathode followerstage is taken, conventionally, from its cathode element through acircuit including capacitor 101 and resistor 106 and is fed through aselector switch hereinafter to be described to the main amplifier of theA. C. amplifier stage. The plate of follower tube 97 is led directly toB| through the voltage dropping -resistor 89. An electrolytic by-passcondenser 103 is also connected to the plate element and to ground.

Preferably, the gain of preamplifier 80, comprising the stages havingtubes 85, '93 and 97, is stabilized by inverse feedback which isobtained by returning a portion of the output signal to the cathode ofthe input tube. This is provided by a resistor 104 which is connectedbetween the cathode element of tube 97 and the cathode element of tube85. It will be noted that loop oscillations are avoided in thepreamplifier stage by the use of only one capacitive interstage couplingnetwork which, in this instance, comprises capacitor 91 and resistor 92.Desirably, `a capacitor 107 is connected -between the plate element oftube 93 and ground to reduce the high frequency response of thepreamplifier. This is made necessary by the presence of noise componentsarising in the input stages of the vibration calibration. By reducingthe bandwidth of the preamplifier, greater accuracy is obtained in theunit.

The main amplifier of vibration calibrator 10 comprises a three sage A.C. coupled amplifier which is gain stabilized by inverse feedback. Thefirst stage is a dualtriode cathode-coupled amplifier which includes `apair of electronic tubes 108 and 109. The tubes, which may be placed ina single envelope for purposes of convenience, are -coupled together attheir cathodes and biased through a cathode resistor 111 by a B- powersupply which, in this instance, is volts. The plate element of tube108.is connected directly to a B+ potential through a voltage droppingresistor 112 while the plate element of tube 109 is connected through aload resistor 113. to the B+ potential through' resistor 112. Anelectrolytic ca'- pacitor 114 is preferably connected to the plateelement of tube 109 and to ground.

The input to the main amplifier stage is taken from a switching' circuitas will hereinafter be described and fed to the control grid element oftube 108. After amplification in the circuit of tube 108 in the firstamplifier stage, the signal is fed to tube 109 through the cathode ofthese tubes such that it appears in phase at the plate element of tube109. The output from the first stage amplifier is then fed to a secondstage amplifier through a resistivecapacitive coupling network includingcapacitor 116 and grid resistor 117 to the control grid `element of anelectronic tube 118 in the next amplification stage. Tube 118 which may`comprise a five element tube has its suppressor grid directly connectedto the cathode and is biased by means of a cathode resistor 119.Resistor 119 is by-passed to ground by an electrolytic capacitor 121.The screen grid element of tube 118 is connected through voltagedropping resistor 122 to the B+ potential While the plate element ofthis tube is connected to the B+ potential through a loading resistor123 and the resistor 122.

The second stage of amplification, like the first stage, issubstantially conventional and operates to develop an amplified signalin its output which is fed to a following or third stagel ofamplification through a resistive-capacitive coupling comprisingcapacitor 126 and grid resistor 127. The third stage of the rnainamplifier is similar to the second stage and includes a pentode typeelectronic tube 128 having its cathode element directly connected to thesuppressor grid and to ground through biasing resistor 12,9, and by-passcapacitor 131. The plate element of tube 128 is connected through loadresistor 132 directly to the B+ potential. The output from the thirdstage of amplification is taken across load resistor 132 and fed to acathode follower isolating stage having electronic tube 133.

A feedback signal for the main amplifier circuit isk obtained through aresistor network including resistors 134, 135, 136 and 137, such thatthe signal output from the third amplification stage including tube 128appears in phase at the control grid element of electronic tube 109.This signal then appears at the plate of tube 109 in opposition to theinput signal fed through the cathode to provide inverse feedback to thecircuit. It will be noted that the loop gain of the amplifier providesapproximately 30 db of inverse feedback. With 30 db of inverse feedback,the amplifier gain is almost equal to the fraction of output signalwhich is fed back to the preceeding stages. This fraction of feedback ispredetermined and fixed by the aforesaid resistor network; the latterbeing proportioned to reduce the output signal by a factor of 1000 sothat the stabilized main amplifier gain is thus made approximately 1000.

In order to avoid loop oscillation at the low frequency of amplifierresponse, the interstage coupling networks comprising capacitor 116 andresistor 117 and capacitor 126 and resistor 127'are proportioned so thattheir halfpower frequency responses are staggered. Similarly, to avoidhigh frequency loop oscillation, the plate load resistors of electronictubes 109, 118 and 128 are chosen in a manner to stagger the half-powerfrequency responses of the individual stages. To further restrict thehigh frequency response of electronic tube 128, a capacitor 138 isconnected between the plate element of electronic tube 128 and ground. 1

The cathode follower stage functions as a buffer between the mainamplifier 82 and the peak-to-peak voltmeterr32 andv thus, serves toprevent the loading of the main amplifier by the voltmeter 32. Moreover,the use of the cathode follower stage provides a convenient terminalarrangement for the connection'of al1-oscilloscope,

if. desired. For the latter, the cathode load resistor of the followerltube 133 is divided into a pair of resistances 139 and 141. Connectedacross resistor 141 are terminals 142.V These terminals permit theconnection of an oscilloscope for observation of the signalwaveformwithf Desirably, theV cathode .Peak-to-peak voltmeter The voltmetercircuit 32 is essentially conventional and comprises Va pair ofcapacitors 171 and 172', each of which is shunted byan electronic diodetube 173 and 174, respectively. Diode 173 and its capacitor 171 form anegative peak detector while diode 174 and its capacitor 172 form apositive peak detector so that these tubes will conduct only when theirrespective peak signals are present. It will be apparent therefore thatthe magnitude of the diode-to-diode D. C. voltage is proportional to thepeak-to-peak value of the incoming A. C. signal. This value is measuredby an amplitude vibration meter 1-76 connected across the diodes`through a pair of resistors 177 and 178. Meter 176 which comprises anammeter, preferably is of a sensitive type so that resistors 177 andv178 may be made sufiiciently large to maintain the low frequencyresponse of the detector; the response being determined by the timeconstant comprising the combination of coupling capacitors 171 and 172and their associated resistors 177 and 178, respectively. ConnectedVacross meter 176 is a capacitor 179 which acts to slow the metermovement or response during operation.

It will be apparent that the current flowing through theloop comprisingresistors 177, 178 and meter 176 will' be additive. In order to cancelany current flow during the interval when a no-signal state is had bythe circuit, a pair of resistors 181 and 182 and a pair of electronictubes 183 and 184 are connected in shunt relation across meter 176. As aresult, the emission current generated by diodes 183 and. 184 will fiowin opposition to the current flow through the meter. The gain of theoutput circuit is controlled through the medium of resistors 186 and 187connected in shuntv relation with meter 176. Desirably, resistor 187 isVariable as Vby a tap 188 so that the sensitivity of meter 176 may beadjusted.

Calbrntr'on lter andrange circuit The calibrate, filter andy rangecircuits are shown in Fig. 3a as comprising the zero calibrate voltagecircuit 33. These circuits include a range switch 200 having two banksof contacts 201 and 202, a calibrate filter switch 204 having fourbanksof contacts 206, 207, 2.08,v and 209, and an electromagnetically drivenswitch with single pole double through contacts otherwise knownV as achopper or vibrator 211, `and a displacement meter 232. The calibratecircuit Vof the instant embodiment serves to convert an incrementalchange in the pick-up D. C. onput voltage related to they standardizingdistance to an A. C. signal and to adjust the indicated amplitude ofmeter 176 to read full scale to correspond with a standard displacementof the pick-up producing the incremental change in the D. C. outputvoltage. As shown in the drawing, the instrument is calibrated byswitching filter switch 204 to its calibrate position and moving rangeswitch 200 to its l0 mil range. Meanwhile, pick-up 14 isphysicallypositioned at a zero set point which is chosen to be a certain fixeddistance, determined by the characteristics of pickup 14 between the endof pick-up 14 and the top surface of the vibration generator or pick-upbeing calibrated. The location of the zerov setpoint for a particularpickup is determined in operation by means of an index mark on Vthedisplacement meter scale.

Connected to the output of pick-up 14 as by a lead 212,

is they mechanical chopper or vibrator 211. The chopper includesan.energizable winding 213, two` sets of stationary 9 contacts 216 and 217,and a vibrating Contact arm 218 which is normally biased into engagementwith ,contact 216. One terminal of winding 213 is grounded while theother terminal thereof is connected through a current limiting resistor219 to the movable arm or contactor 221 of Contact bank 209. When filterswitch 204 is in calibrate position, arm 221 engages the first or leftterminal of contact bank 209 which in turn is connected to analternating current power source through lead 222. It

will be apparent therefore that when filter switch 204 is placed incalibrate position, winding 213 will be energized by an alternatingcurrent source and that contact arm 218 will vibrate between contacts216 and 217. The remaining terminals of contact bank 209 are eachconnected to a B-lpotential source through line 225 for reasons whichwill soon become apparent.

Stationary contact 216 of chopper 211 is directly connected to lead 212,while stationary Contact 217 of chopper 211 is connected to a variabletap of a calibratezero potentiometer 223. One terminal of potentiometer223 is connected through a resistor 224 to ground while the otherterminal of the potentiometer is connected through a resistor 226 to asource of regulated B| potential.

The calibrate-zero potentiometer circuit or zero-balance circuit servesto apply a reference D. C. voltage to contact 217 of the chopper. Byadjusting potentiometer 223,

the potential of this D. C. voltage may be varied to be equal to the D.C. output voltage of pick-up 14 when'the pick-up is positioned in itszero-set point. Consequently, when properly adjusted, as vibrating arm218 engages contact 217, and the potential on this contact is adjustedto be equal to the output potential as applied from pickup 14 to contact216, the square wave voltage output from chopper 211 will be zero.Therefore, meter 176 will indicate a zero or minimum setting oramplitude. After this initial setting, pick-up 14 is moved toward thesurface 24 of vibrator 12 a predetermined standardized amount.Desirably, this distance is set at exactly .02 inch by precise measuringapparatus such as, for example, a conventional micrometer. So that theprescribed distance may be accurately and repeatedly set through theapparatus alone, the output of pick-up 14 is also fed through line 231,a resistor 235, and calibrate contact or terminal of contact bank 206,to a sensitive displacement meter 232. The meter, which is of theammeter type and responsive to the D. C. component of the pick-up outputvoltage, is provided with suitable markings on the meter face forvisually indicating the zero set and operate points to enable theoperator to set pick-up 14 the desired distance from the surface 24whose amplitude of vibration is to be measured. In other than calibrateposition, the meter is removed from the circuit and grounded toeliminate any spurious signals which may be generated by the meter whensubjected to vibration. This grounding operation is performed byconnecting the remaining terminals of contact bank 206 to ground. Ifdesired, a resistor 233 and switch 234 may be placed in parallel withmeter 232 to permit exciter tests for checking the instrument.

Contact bank 207 is used only when filter switch 204 is placed incalibrate position. Connected to the first or calibrate terminal of thisbank and to ground is a parallel connected capacitor 237 and resistor236. The function of capacitor 237 is to by-pass to ground the switchingtransients which occur during make and break of the vibrator contactsand therefore to facilitate the bucking out or adjustment of thecalibrate-zero function.

Resistor 236 and capacitor 237 are inserted to shunt with attenuatorresistor 238 in the calibrate position of filter switch 204. Resistor236 in this circuit serves to increase the attenuation of the decadedivider associated with the range switch, as hereinafter disclosed. Thisincreased attenuation acts to compensate for low-frequency distortion ofthe Calibrating waveform. Moreover, re-

sistor 236 serves to compensate' for any errors that may be causedby theimpedancepof the zero-balance voltage source. Since this source ofimpedance is in series with the decade attenuator resistance during onlyhalf the chopper cycle, the peak-to-peak amplitude of the chopper signalwill necessarily be less or may be less than the difference between theprobe voltage and the zero balance voltage.

The filter circuit of the present embodiment is shown as 'beingconnected in contact bank 208 of filter switch 204 'and comprises aseries of Ycapacitors of decreasing size that are switched in serieswith capacitor 227 to restrict the low frequency response of thevibration measuring system at this point. In calibrate and normalposition of filter switch 204, only capacitor 227 is inesrted in thenetwork of the decade resistance divider, indicated vby contact bank202. In the 20 cycle, 200 cycle and 2000 Cycle high-pass ranges,capacitors 228, 229, and 230,1respectively, are connected in series withcapacitor 227. These capacitors serve to restrict the low frequencyresponse of the amplitude response of the vibration calibrator so as toprovide a fiat amplitude response only above the indicated frequencies.

The range of peak vibration amplitude which is measurable by thevibration calibrator is covered by four decade settings of range switch200. These ranges are .01, .1, 1 and 10 mils full scale, respectively.It will be noted that in the .0l mil range, the preamplifier is insertedin tandem with the main amplifier 82. This may be readily seen from thecircuit as traced from line 212 leading from the pick-up 14, contact arm218, capacitor 227, the filter switch, to the .01 terminal of contactbank 201, the assocaited switching arm, lead 86 which extends to thegrid of electronic tube of preamplifier 80, the output of electronictube 97, contact bank 202, and lead 241 to the control grid Iofelectronic tube 108 of main amplifier 82. By switching the preamplifierin the circuit on the .01 mil range, the preamplifier serves to providea gain of 10. However, upon switching to the .l mil range, thepreamplifier is removed from the circuit, leaving only the mainamplifier ahead of the output circuit. In order to obtain the remaining2 decade ranges l and 10, a resistance divider comprising resistances246, 247 and 238 are provided in the circuit of contact bank 202. AsWill hereinafter be described, switching of the filter and rangeswitches 200 and 204 provides a control over the range and width offrequencies as well as the range of vibration amplitude to be measured.

Operation Before vibration readings may be taken, the vibrationcalibrator is standardized with respect to the metallic surface of theVibration generator to be calibrated. In Figure 1, surface 24 comprisesthe vibrating portion of mechanical vibrator 12. It should be noted thatfor accurate measurements, the Vibrating surface should comprise anon-magnetic metallic surface having a minimum diameter of Ms of an inchand a thickness of not less than .01inch. Moreover, the metallic surfaceshould have a resistivity not exceeding 8X10?6 ohms per centimeter. Theinstrument is energized through a suitable power switch 300 positionedon housing 17 in Figure l. After a warm-up period wherein the units ofthe vibration calibrator are energized for a period of time, rangeswitch 200 is switched to its l0 mil position, and filter switch 204placed in its calibrate position. The pick-up spacing is then adjustedrelative to vibrating surface 24 to its zeroset point as determined by adesignated mark on the displacement meter. Calibrate zero potentiometer223 is, then adjusted until a minimum reading is obtained on voltmeter176.

It will be noted that during this initial adjustment, winding 213 ofchopper 211 is energized by an alternating current through line 222 andthat contact arm 218 lis"vibratingbetween stationary contacts 2.16 and217.:

Also, surface 24 is stationary. Therefore a quiescent DTC., a signalfrom pick-.up 14, .is nfed through line 212 to arm 218, during one rhalfof the chopper switching cycle. On the other half of the switchingcycle, when contact arm 218 engages contact 217, the adjustable yD.-C.voltage developed by thecalibrate zero potentiometer from the regulatedB+ power supply connected thereto, appears on contact 218. Theincremental signal i. e. the square wave of voltage with a peak to Vpeakamplitude equal to the difference between the DFC. voltages impressed oncontacts 216 and 217, .developed on contact 218 passes through capacitor227, calibrate terminal of contact bank 208, contact bank 201,preamplifier 80, main amplifier 82, and voltmeter circuit 32. vIt willbe apparent, therefore, that by adjusting the calibratezeropotentiometer 223, the regulated potential applied to the circuit may bemade to provide a null or zero point reading on meter 176 to correspondto the zero=set point of the pick-up. This zero set position is acertain fixed distance from surface 24, and is determined in operationby a marking on the displacement meter 232.

Upon adjustment of the zero-set point, piek-up 14 is moved an exactamount toward surface 24 such as, lfor instance, .02 inch, to provide anaccurate standardizing displacement. The resulting peak to peak voltageamplitude appearing on chopper contact 218 corresponds to the amplitudeof a signal from the carrier detector that would be produced by avibratory motion of surface 24 having a peak to peak vibration amplitudeequal to the measured standardizing displacement. The indicationappearing on displacement meter 232 is then observed and preferablymarked so that the marking on the meter may be used to accuratelyposition any future probe adjustments to the set displacement.Standardization of the vibration calibrator is completed by adjustingthe gain set control 188 of voltmeter circuit 32 to provide a full scalereading on meter 176. It will be obvious that after making the aboveadjustments, whenever the A. C. component of the modulated signaldeveloped across pick-up 14 reaches an amplitude of .02 inch or anyother amplitude within its range, meter 176 will provide a directreading of this amplitude.

After these adjustments have been made, the vibration calibrator isstandardized and is prepared vfor calibration of vibration generator 12by adjusting the spacing between the surface of vibration generatorsurface 24 and the pickup 14 so that the displacement meter reads at apredetermined operate point. This insures that vibration measurementswill be made over a relatively at portion of the sensitivitycharacteristic of pickup 14. In making a calibration of generator 12,filter switch 204 is placed in normal position. It is -noted'that ininstances where the measurements of generator 12 are to .be taken over arestricted bandwidth, filter switch 204 is placed in one of its otherpositions which are designated by specific frequency values. Thesefrequency designations indicate the frequencies below which the responseof the instrument to vibrations is suppressed. Also, range switch 200 isset in a selected position such as .l mil; the position selecteddepending .upon the anticipated amplitude of vibration to be measured.peak amplitude of vibration as indicated byy meter 176 is obtained bymultiplying the vibration meter lreading by the setting of range switch200. f

With the above typical settings, the specific operation in measuring andCalibrating the vibrations lofsur-face 24 f Upon energization, carrieroscillatorA 26- is as follows: y operates to generate a circulatingcurrent in its tank circuit at a frequency determined by the constantsof the tank circuit. Should the product of theV current and fre-v quencyvary in the tank circuit, an error signal isdeveloped in the regulationcircuit29, amplified, and appliedV to the carrier oscillator circuit asa correcting signal.

tank circuit, is energized by the circulating current and In any event,the

induces a voltage in winding 22 which is modulatedby the vibrations Aofsurface 24. This voltage is detected by detector 73 and fed to arm 218of electro-mechanical chopper 211 through line 212. It is noted thatfilter switch 204 is in `normal position and as such, winding 213 isenergized by the D. C. potential source and arm 218 is in stationaryengagement with contact 216. The signal therefore passes throughblocking capacitor 227, normal terminal of contact bank 208, to the .lmil terminal of contact bank 202. From there, the signal passes throughline 241 to the first stage of main amplifier 82 where the signal isamplified and fed through a cathode follower stage including tube 133.The output of this tube is taken across the cathode load resistor 139,141 and fed to the voltmeter circuit 32 wherein tubes 173 and 174 detectthe peak to peak amplitude signal. The peak amplitude of a sinusoidalvibration is then indicated by meter 176, since, the meter scale is socalibrated. Also, since the vibration calibrator has been standardizedby means of a static displacement of .02 inch, and meter 176 has beenadjusted to provide a full scale reading peak amplitude reading for thisstandardizing displacement through the medium of gain control 188, thepeak amplitude of the sinusoidal signal as indicated by meter 176 willbe the amplitude of the vibrating surface.

It will be vnoted that in the instances where range switch 200 Vis-setto its .01 mil range, the signal is fed from filter switch 204 throughcontact bank 201, through line 86 to the first amplifier tube 85 ofpreamplifier 80. This signal is then amplified in the preamplifier andfed through line 225 to contact bank 202, line 241, and then to the mainamplifier. As the signal is passed through preamplifier 80, a gain of l0is obtained.

As best shown in Figures 4, 5 and 6, the preferred lstructuralembodiment of the transducer or pick-up 14 includes a yoke or housingmember 400 which is adapted to be suitably mounted in operative relationto the vibrating `surface 24. The yoke member, which may be rectangularin construction, is provided with an elongated horizontal bore 401 inits upper portion (as viewed in Figure 4*) for adjustably receivingtherein an elongated cylindrical pick-up barrel 402. Desirably, barrel402 is formed with an enlarged threaded portion 403 along its left outersurface to permit the lateral movement thereof in the yoke member 400.Fitted within a downwardly extended slot 404 formed transversely kto thebore 401 in yoke member 400 is an annular adjustment nut 406. Theadjustment nut 406 is provided with Va threaded bore 407 therethroughwhich is adapted to be threadably received by the threaded portion 403of barrel 402. Because of the narrowly spaced, parallel surfaces orsides of slot 404, the nut is restrained from lateral movement so thatrotation of nut 406 serves to impart a lateral movement to pick-upbarrel 402. For convenience of hand adjustment, the outer periphery ofadjustment nut 406 is preferably knurled as at 408.

The right end reduced portion of barrel 402 is supported within bore 401by a suitable sleeve 411 fitted within the bore. Formed through yokemember 400, transversely to bore 401 and intersecting the lower edge ofsleeve 411 is a bore 412. The bore serves to carry a cylindrical lockingnut 413 and a cylindrical locking bushing 414;,each of which members isprovided with a bore therethrough for receiving a locking bolt 416. Bolt416, which has a wing nut 417 on one end for hand adjustment purposes,extends through the bore in locking bushing 414 and threadedly engagesthe locking nut 413. It will be apparent from this construction that thebarrel may be readily adjusted in a lateral directionV (as viewed inFig. 4) by rotation of adjustment nut 406 and that by .tightening wingnutV 417, of bolt 416, Ythe locking bushing 414Y may be clamped againstbarrel 402, through V,intersected'sleeve y411, to retain the. barrel inadjusted position within the yoke. VIf desired, a lock-screw 418 may beused to engage the threaded portion 403 of barrel 402 to lock the barrelin position.

The pick-up barrel 402 supports and shields the electrical components ofthe transducer and generally cornprises an elongated tubular memberhaving its left end closed except for a passageway 419 formed thereinfor the entrance of electrical wiring. Desirably, a wire-clamp 421 isprovided on the extreme left end to secure the wire connections in thepick-up against possible damage by pulling or other physical force.Carried within the barrel and secured thereto by a threaded bolt 422 isa Bakelite insert 423 having its right end portion suitably formed toconform with the internal circular configuration of barrel 402. The leftportion of insert 423 is cut away and provided with a pair of terminallugs 424 which serve to support the diode detector 73 (Fig. 3b).

Fitted within a cut-away portion 426 on the extreme right end of barrel402 is a copper sleeve lining 427. The sleeve lining acts to reduce orminimize eddy-current and hysteresis losses in the barrel. The extremeouter end of insert 423 is provided with a first reduced portion 429 forsnugly receiving the secondary windings 22 of the pick-up 14. Also, asecond extended cut-away portion 428 is provided on the right portion ofthe insert 423 for receiving an insulating tubing 431 such as Pyrex orthe like. The tubing serves as a coil-form for primary winding 21 and toinsulate the primary winding 21 from the secondary winding 22.

It will be noted that the characteristics of the pick-up are determinedby the particular characteristics of the winding. For instance, therange of linear operation of the transducer Iis a function of thewinding configuration, that is, whether they are coaxial, circular andcoplanar, an'd, further, the ratio of the diameter of the secondarywinding to the Idiameter of the primary winding. For a given turnsratio, the range of linear operation is increased as the ratio ofwinding diameters is decreased but with a resultant decrease intransducer sensitivity. Therefore, the choice of transducer dimensionsis determined by the minimum sample size and the range of Vibrationamplitude which is to be measured.

Obviously many modifications and variations of the present invention arepossible in the light of the above teachings. It is therefore to beunderstood that within the range of the appended 'claims the inventionmay be practiced otherwise than as specifically described.

What is claimed is:

1. In an apparatus for measuring the vibration characteristics of avibration means comprising, in combination, a mutual-inductance pick-upmeans adjustably positioned adjacent said vibration means and operativeto produce an output signal proportional to the amplitude of vibrationof said vibration means, regulated energizing means for said pick-upmeans, switching means, ampliiier means, and indicator means seriallyconnected to `said pick-up means for amplifying and indicating thequantity of said output signal, and means for standardizing saidindicating means in accordance with said vibration means, saidstandardizing means compris-ing a chopper means and a reference voltagesource, said chopper means having two stationary contacts and a movablearm electromagnetically operable to alternately connect said contacts tosaid indicator, one of said contacts being connected to said pick-upmeans and the other to said reference voltage source, said chopper beingoperative to provide in its output, signal portions of said outputsignal and a reference voltage from said voltage source, and means foradjusting the initial indication of said indicator means in accordancewith said signal portions.

2. In an apparatus as defined in claim 1 but further including adisplacement indicator means connected to said pick-up means forproducing an indication of the physical displacement between saidvibration means and pick-up means, and a signal attentuation meansconnected between one of said chopper contacts and said indicator.

3. An apparatus for Calibrating the vibration characteristics of avibrating body by `comparison with a standard displacement, comprisingin combination a mutual inductance pick up means having a substantiallyliat sensitivity Icharacteristic over a predetermined range of vibrationamplitudes for producing a signal output proportional to the vibrationamplitude of said body, means for energizing said pick up means, meansfor adjustably positioning said pick up means adjacent said vibratingbody, an indicator, means for standardizing said apparatus over saidpredetermined range on said indicator when :said vibrating body is 'in aquiescent condition, amplitude and frequency range selective meansserially connected to said pick up, and A.C. amplifying means connectingsaid selective means to said calibrated indicator.

4. An apparatus as recited in claim 3 wherein said means forstandardizing said indicator comprises a source of reference voltage,chopper means disposed between said pick up means and said indicator foralternately connecting the output of said pick up means and saidreference Voltage to said indicator, means for adjusting said referencevoltage whereby said indicator reading is zero when said pick up meansis at a predetermined distance from said quiescent vibrating body andmeans for adjusting said Iindicator whereby a full scale reading isobtained when said pick up means is moved an exact distance from saidpredetermined distance.

5. An apparatus for accurately measuring vibration amplitudes of atransducer over a wide range of amplitude and frequency 'comprising amutual inductance pickup means positioned a distance from saidtransducer determined by the characteristics of said pick up and therange of amplitudes to be measured, regulated means for energizing saidpick 'up means, means for directly comparing the output voltage of saidpick up means with a voltage obtained from a predetermined staticdisplacement, said means comprising serially connected switching means,range and frequency selective means, gain stabilized A.C. amplitermeans, and an indicator, said switching means cooperating with a sourceof reference voltage to convert the D.C. output of said pickup means'due to sai-d static displacement into an A.C. voltage whereby saidindicator is standardized in accordance with said static displacement.

References Cited in the file of this patent UNITED STATES PATENTS1,842,190 Ochse Ian. 19, 1932 2,234,056 Moore Mar. 4, 1941 2,438,506Ladrach Mar. 30, 1948 2,629,004 Greenougli Feb. 17, 1953 2,648,979Cornett Aug. 18, 1953 2,661,622 Severs Dec. 8, 1953 FOREIGN PATENTS201,985 Great Britain Aug. 1, 1923

